Method for making identity cores for ultrasound seals

ABSTRACT

The invention concerns a method for making identity cores for ultrasound seals. Each core comprises a chip consisting of a brazed stack of metal disks  1  bearing at least one notch  3 , the resulting identity being defined by the number, the size and the random angular position of the various notches in the stack and by the unpredictability of the brazing. The invention is characterized in that the notched disks  1  provided with a central hole  2  are first stacked in large numbers in a tubular metal sheath  4  with a circular cross-section corresponding to said disks  1 . The stack is then axially compressed in the sheath  4  and a brazing bead  7  is inserted in the channel constituted by the assembly of aligned central holes  2 . After brazing in a vacuum oven, the two ends of the sheath are eliminated and the resulting column is segmented perpendicularly to its axis into a series of individual identity chips  8  of predetermined thickness. Such a chip is finally associated by brazing with a metal block  13  used as delay line to constitute the core. Identity control can be combined with integrity control by associating an integrity rod  10  to  12  with said block  13 , whereby with one reading both identity and integrity can be ascertained.

BACKGROUND OF THE INVENTION

The present invention refers to a method of making identity cores forultrasound seals, each core comprising a brazed stack of disks eachpresenting at least one notch, the identity being at least in partdefined by the random angular position of the various notches in thestack of disks.

Such a core is described in the patent document EP 0 638 250 A.

These cores are intended for being incorporated in seals or markers forcontrolling and handling dangerous materials, especially radioactivematerials, in order to allow their control and to contend with frauds.

According to the terms of the Nuclear Non-Proliferation Treaty, anytransport or storage of radioactive materials should be subjected to apermanent control by international inspectors. In order to comply withtheir mission the inspectors should be able to seal recipients and toverify the identity and integrity of such seals.

In the above cited documents bolts are described which insure locking ofa recipient and which are supplied with an identity core. By applying anultrasound reader to the head of the bolt, the identity can be read, andsimultaneously it can be verified if the bolt has been manipulated sincethe most recent prior verification.

It is known to make use of random phenomena for the definition of theidentity in order to avoid any fraudulent tentative to copy a seal. Thusthe document cited above proposes to make identity cores by stacking acertain number of notched disks in such a way that, after a convenientassembly of the stack by soldering, the identity is defined at least inpart by the random position of the notches in the stack, that means bythe cavities which after soldering generate ultrasound echoes. Theidentity is thus defined after soldering a random assembly of disks tobecome one block, the random nature being based on their individualfabrication.

SUMMARY OF THE INVENTION

These cores may be combined with an integrity rod having a zone ofpreferred breaking in case of an integrity violation. By applying anultrasound reader to a bolt incorporating such a core, the ultrasoundresponse which is registered corresponds on the one hand to the identityof this seal and on the other hand, through the integrity status of therod, to the integrity status of the bolt.

The present invention concerns the method for making identity coreswhich are intended to be used in such seals or others. In fact, it hasbeen noted that the individual manufacturing of the cores is complex dueto the large number of required cores (after each opening of a recipienta new core is necessary) and due to the small size of the cores (12 mmin diameter, 5 mm in height) and especially due to the qualityrequirements. For example, an absolute plane surface of the disks mustbe ensured, the thickness of a disk being in the range of onemillimeter.

The method according to the invention is defined by the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by means of apreferred embodiment and the attached drawings.

FIG. 1 shows a disk which is part of the core made according to theinvention.

FIG. 2 shows an axial section through a stack of disks ready forsoldering in the frame of the method according to the invention.

FIG. 3 shows a step subsequent to the soldering, that means the step ofsubdividing the stack into individual chips.

FIG. 4 shows such a chip from above which has been modified in view of acombination with an integrity rod.

FIG. 5 shows a cut view through the combination of a core as modifiedaccording to FIG. 4 and an integrity rod during the soldering of thesetwo elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows from above a disk made of metal sheet. Several such disksmake up an identity core. According to this embodiment each disk 1 ofsubstantially circular shape and of a thickness of 1 mm comprises acentral hole 2 (diameter 3.6 mm), and laterally at least one notch 3which extends radially from the periphery towards a point close to theborder of the central hole 2. The width of this notch is for example 1.5mm. Such disks can be realised by stamping. It should however be ensuredthat these disks have perfectly plane surfaces and no beards.

If disks of a unique thickness having only one notch were used, then thevariety of obtainable identity cores would be rather small. However,this variety can be increased at constant detection quality by providingtwo, three or even four notches and/or by providing disks of variousthicknesses, the different types of disks being then randomly combined.The angle between two adjacent notches should be large enough in ordernot to weaken unduly the mechanical stability of the disks. This angleshould for example be larger than 40°, preferably larger than 60°.

Here-under cores will be described which are composed of disks havingvarious thicknesses between 0.9 and 1.3 mm, but the invention is notlimited by these figures.

After the manufacture of the disks by stamping, disks are stacked inlarge number without pre-established order in a tubular sleeve 4 asshown in FIG. 2. The sleeve presents at one end a hole 5 and can beobturated at the other end by a threaded plug 6. The dimensions of thissleeve 4 are selected in such a way that its inner volume can be filledby a given number of disks, for example 50 disks, and that this stackcan be compressed by fastening the plug 6.

Thereafter, a solder material stick 7 is inserted through the hole 5 ofthe sleeve into the central channel composed from the central holes 2 ofall the disks.

All the components (sleeve, plug, disks) should have a perfect surfacequality prior to the assembling.

After having received a compressed stack of disks and a soldering stick,the sleeve is entered into an oven. As known form the state of the art,the interior of the oven can be subjected to a vacuum and/or to an inertatmosphere. Moreover, the increase and decrease of the temperature canbe programmed according to the necessities.

During this soldering phase, the solder material penetrates bymoistening into all the metal-metal interfaces and moistens all thewalls, even those of the cavities constituted be the notches 3 of thedisks. As the case may be, a small quantity of soldering material couldbind a residual particle at the wall thus causing the creation of anadditional random factor. After the soldering phase the disks aresecured one with respect to the others in a random angular position andmake up a compact block.

After withdrawal from the oven the bottom of the sleeve and the plug areeliminated, for example on a lathe, and the remaining column is dividedperpendicularly to its axis into a series of chips of predeterminedthickness, this cutting being for example achieved by electro-erosion.

FIG. 3 shows this column according to an axial cut view after itssubdivision into nine chips such as chip 8. The hight of each chip is5.2 mm. The cutting planes are randomly located with reference to theinterface between two disks. Each disk presents an individual identitywhich depends on the angular position of the metal disks which itcontains, this identity being detectable by an ultrasonic readerscanning along a circular path of intermediate diameter between that ofthe hole 2 of the disks and the periphery of the chip. By this means,such a chip constitutes in combination with a delay line 13 (see FIG. 5)an identity core which can be incorporated by mounting or solderingmeans into a bolt which for example safely seals a barrel filled withradioactive waste, the random character of the structure of the coreensuring the protection against copying.

If besides the identification function a seal or a barrel or otherobject to be controlled must also be protected against a non-authorizedmanipulation, the core should be combined with an integrity rod whichbreaks during such a manipulation. To this end a groove is cut along ageneratrix line of the soldered column as shown in FIG. 2. After thesubdivision into chips each chip 8′ as modified presents with respect tochip 8 in FIG. 3 an additional, open notch 9.

This notch 9 receives freely an integrity rod 10 which is an extensionof the metal block 13, the latter acting as a delay line for ultrasoundwaves after having been soldered to the identity chip 8′. The integrityrod 10 presents a portion 11 of reduced cross section for privilegedbreaking which will break if a traction, torsion or shear force (or acombination of these forces) is applied. This rod is connected in thefinal assembly by its anchoring tail 12 to a mechanical element (notshown) inside the bolt to be protected.

The block 13 (delay line) with its integrity rod 10, as the case may be,is inserted into a soldering sleeve similar to the soldering sleeve 4.The plug 15 of this sleeve comprises a central cavity 16 for receiving asolder material stick. It can be threaded into the sleeve in order tocompress the block 13 and the identity chip 8 or 8′ underneath. Thisassembly is soldered according to a method similar to that which hadbeen described in accordance with FIG. 2. However, this time thesoldering will no more penetrate into the stack but simply securestogether the chip 8 or 8′ and the block 13 with the sleeve 14. It shouldbe avoided here by classical means such a hydrophobic product that thesolder material penetrates into the space between the wall of the notch9 and the rod 10. The surfaces moistened by the solder material aremarked by a fatty line 17.

The last step of making this combined identification and integrity coreconsists in separating the head of the sleeve with the plug 15 from therest by cutting perpendicularly to the axis of the core along a line 18at the upper level of the block 13 corresponding to the exactly desiredthickness for the delay line. In the same way the portions of the sleeve14 which have become useless are withdrawn, on the one hand by cuttingalong a line 19 perpendicularly to the axis at the lower level of thechip 8′ and on the other hand by reducing the diameter of the sleevedown to a cylinder 20 thus retaining only a sheath of about 1 mmthickness around the assembly consisting of the block 13 and the chip8′. The surface for reading the identity and integrity state of the sealis then constituted by the cut surface 18. By moving a read head along acircular line coaxial to the core and situated half-way between thecentral hole 2 and the periphery of the core, a response is obtainedwhich for at least 80% of the circular path contains informationsrelating to the identity of the core and for the rest informationsconcerning the integrity of the rod. These two types of response areclearly differentiated mutually due to the presence of the integrity rodin the additional notch 9, said rod reflecting, contrary to the notches3 in the disks, no echoes as long as it is not broken.

The invention is not limited to the embodiments as described above indetail. In particular, the identity core can be applied alone for simplydefining in an unambiguous manner the identity of an object to which itis associated or incorporated.

In addition, it is clear that the size and shape indications suppliedabove only serve to illustrate an embodiment and can be modified ifnecessary without escaping from the scope of the attached claims. As faras the materials are concerned which are used especially for the disksand the solder and as far as the temperature and treatment time for thesoldering are concerned, reference is made to the abundant literature inthis field.

What is claimed is:
 1. A method of making identity cores for ultrasoundseals, each core consisting of a delay line and a soldered stack ofmetal disks each presenting at least one notch, the identity beingdefined by a random angular position of different notches in the stack,characterized in that many notched disks each comprising a central holeare firstly stacked in a tubular metal sleeve of circular innercross-section corresponding to the size of the disks, the stack isaxially compressed in the sleeve and a soldering stick is introducedinto a channel constituted by an assembly of the central holes which arealigned, a soldering is performed in a vacuum oven, after elimination oftwo end portions of the sleeve, a column obtained in this way is cutperpendicularly to its axis into a plurality of individual identitychips of predetermined thickness, and said core is obtained by solderingto such a chip a metal block which acts as delay line.
 2. A methodaccording to claim 1, characterized in that the notches present anelongate shape and extend radially from the periphery of the disktowards a diameter which is slightly greater than the diameter of thecentral hole.
 3. A method according to claim 1, characterized in that,the number of notches in a disk being greater than one, the anglebetween any two respective adjacent notches is at least equal to 40%. 4.An identity core made according to the method as defined in claim 1,characterized in that it is part of a soldered identity and integrityassembly and comprises an additional notch made after the soldering ofthe stack and extending over the entire thickness for permitting anintegrity rod to pass therethrough which is part of the metal block andpresents a zone of privileged breaking if the integrity is violated,this violation being detectable by ultrasound waves during a read-outoperation of the core identity.
 5. A method according to claim 1,characterized in that, the number of notches in a disk being greaterthan one, the angle between any two respective adjacent notches is atleast equal to 60%.